Ultra-broadband mid-infrared supercontinuum-based spectroscopy for greenhouse gas monitoring at wastewater treatment plants

Ultra-broadband spectroscopy in the mid-infrared (MIR) wavelength range, where most molecular species have strong, distinct absorption features, has a great potential for gas sensing applications. Novel MIR supercontinuum (SC) sources excel in their ability to provide broadband light together with a high spatial coherence. Using this unique combination of properties, we have recently demonstrated the potential of MIR SC sources in combination with a tailor-made Fourier Transform Spectrometer (FTS) and a multipass absorption cell for multispecies trace gas detection [1]. Moreover, a novel application is to utilize the spatial coherence of the source to monitor outdoor (greenhouse) gas concentrations through free space. In open-path absorption spectroscopy, the light beam is guided over an (outdoor) path, instead of sampling the gas in a cell. The concentrations of the gases of interest are integrated over this path, which is useful for detecting greenhouse gases in an area or around an emission source. Wastewater treatment plants are a known source of greenhouse gas emissions of CO₂, CH₄, and N₂O. However, little is known about the fluctuation in the emission rates of these gases over time and their relation to internal and external factors.

We have developed a new, transportable instrument for open-path absorption spectroscopy, which comprises of a unique, novel SC source with an ultra-broad spectrum from 2 – 11.5 µm with ~3 W output power and a custom-built Fourier transform spectrometer [2]. The beam of the SC source is sent over an open path to a cubic retroreflector, where it is reflected back to the FTS. Using the spatial coherence of the beam, extensive, outdoor optical paths can be achieved, while the broad spectrum enables simultaneous detection of many different gas species.

We present the results of field measurements at a wastewater treatment plant where we monitored the concentration of greenhouse gases (CH₄, N₂O, and CO₂) and other trace gases (e.g., NH₃ and CO) simultaneously in the atmosphere surrounding the aerobic tank of the plant (Figure 1).  We will shed light on the perspective of this novel instrumentation for greenhouse gas monitoring around emitting sources, as it is providing reliable data for modelling studies on the dynamics of the emissions of wastewater treatment plants and other sources.

Figure 1: Satellite image of the aerobic tank of the wastewater treatment plant with the beam path over the tank (in red). Left insert: Retrieved concentrations of methane using the open-path instrument (in black) and using a validation instrument (in red, measured at point marked “x” in the photo). Right insert: Close-up of the open-path instrument.

 

[1] M. A. Abbas, K. E. Jahromi, M. Nematollahi, R. Krebbers, N. Liu, G. Woyessa, O. Bang, L. Huot, F. J. M. Harren, and A. Khodabakhsh, "Fourier transform spectrometer based on high-repetition-rate mid-infrared supercontinuum sources for trace gas detection," Opt. Express 29, 22315-22330 (2021).

[2] R. Krebbers, K. van Kempen, F. J. M. Harren, S. Vasilyev, I. Peterse, S. Lücker, A. Khodabakhsh, and S. M. Cristescu, “Ultra-broadband spectroscopy using a 2–11.5 µm IDFG-based supercontinuum source”, Optica Open. Preprint. DOI:10.1364/opticaopen.24967692.v1.